Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 26888)
+++ src/wp-includes/media.php	(working copy)
@@ -655,14 +655,11 @@
 	if ( ! empty( $atts['id'] ) )
 		$atts['id'] = 'id="' . esc_attr( $atts['id'] ) . '" ';
 
-	$caption_width = 10 + $atts['width'];
+	$caption_width = $atts['width'];
 
 	/**
 	 * Filter the width of an image's caption.
 	 *
-	 * By default, the caption is 10 pixels greater than the width of the image,
-	 * to prevent post content from running up against a floated image.
-	 *
 	 * @since 3.7.0
 	 *
 	 * @param int $caption_width Width in pixels. To remove this inline style, return zero.